Many additive systems for improving wet-end drainage and retention have been disclosed in the prior art including those employing combinations of colloidal silica and organic polymers. Such systems are among the most efficient now in use but they are also among the most expensive and there is a continuing need to improve additive performance while reducing additive cost. Consequently, it is a primary object of this invention to provide a method whereby additive cost can be significantly reduced while at the same time increasing additive performance.
This invention employs as a retention and drainage aid water soluble polyaluminosilicates microgels formed by the reaction of polysilicic acid with an aluminum salt, preferably an alkali metal aluminate. They consist of aggregates of very small particles having a high surface area, typically about 1000 meters.sup.2 /gram (m.sup.2 /g) or greater and an alumina/silica mole ratio or content greater than about 1/100 and preferably between about 1/25 and 1/4. Their physical structure is believed to form particle chains and three dimensional networks or microgels.
The water soluble polyaluminosilicate microgels and a process for making them are taught in co-pending U.S. Application to John Derek Rushmere CH-1554A, a Continuation-in-Part of CH-1554, both of which are incorporated herein by reference.
The polyaluminosilicates thus formed provide improved operating benefits over the aluminated colloidal silicas of the prior art in papermaking. Such prior art commercial aluminated colloidal silicas used in papermaking consist of larger, non-aggregated particles with a surface area of about 500-550 m.sup.2 /g, a surface acidity of 0.66 milliequivalents per gram (meq/g) or less, and an alumina/silica mole content of about 1/60.
It is known that amorphous water insoluble polyaluminosilicates can be formed by the reaction of alkali metal polysilicates with alkali metal aluminates. Such polyaluminosilicates or synthetic zeolites have found use as catalysts, catalyst supports and ion exchange materials. Also, it is known that the particles in colloidal silica sols can be surface aluminated by aluminte ions to form a coating of polyaluminosilicate as disclosed in the book "The Chemistry of Silica" by Ralph K, Iler, John Wiley & Sons, NY, 1979, pp. 407-410.
U.S. Pat. No. 4,213,950 discloses an improved process for the preparation of amorphous, water insoluble polyaluminosilicates by the reaction of alkali metal aluminates with aqueous polysilicic acid at pH 2-4. The disclosure stresses the use of true solutions of polysilicic acid not appreciably crosslinked and distinguished from colloidal solutions, suspensions, dispersions and gels.
The new water soluble polyaluminosilicate microgels employed in this invention have unique properties and characteristics. They are formed over a wide pH range of 2-10.5 by the reaction of aqueous solutions of partially gelled polysilicic acid and an aqueous solution of an aluminum salt, preferably an alkali metal aluminate, followed by dilution of the reaction mix before gelation has occurred in order to stabilize the polyaluminosilicate microgels in an active form. Alternatively, the water soluble polyaluminosilicate microgels may be produced by dilution of the polysilicic acid stock before mixing with the alkali metal aluminate. The water soluble polyaluminosilicates so produced are distinct from the amorphous polyaluminosilicates and polyaluminosilicate coated colloidal silicas of the prior art in that they have a very high surface area, typically 1000 meter.sup.2 /gram (m.sup.2 /g) or greater and surprisingly a very high surface acidity, typically 1 meq/g or greater. The alumina/silica mole ratio or content is generally greater than about 1/100 and preferably between about 1/25 and 1/4. Their physical structure is believed to consist essentially of aggregates of very small particles of silica, surface aluminated, formed into chains and crosslinked into three-dimensional networks or microgels. Some colloidal silica and colloidal alumina particles may be present with the polyaluminosilicate microgels.
The water soluble polyaluminosilicates microgels used in this invention are believed to derive their structure from the polysilicic acid stock formed initially by an appropriate deionization or acidification of a dilute alkali metal polysilicate, for example Na.sub.2 O.3.2SiO.sub.2. Such polysilicic acid stock, also known as "active silica" consists, according to Iler in the above cited text, p. 174 and 301-303, of very small 1-2 nanometer (nm) primary particles which are aggregated into chains and three dimensional networks or microgels. Such networks, when converted to aluminosilicates by reaction with sodium aluminate exhibit a considerably greater efficiency in flocculating fiber and filler fines than larger non-aggregated aluminated silica particles particularly when employed with a cationic polymer, such as cationic starch, cationic guar or cationic polyacrylamide. The greater efficiency in flocculation is believed to result from both the increased effectiveness of the microgel structure in locking together or bridging pulp and filler fines and also from the high surface acidity more effectively completing charge neutralization reaction with the cationic components.
The water soluble polyaluminosilicates have a wide range of application to different papermaking stocks including those containing bleached kraft pulp, groundwood pulp and thermomechanical pulp. They may also be used for the clarification of white waters and the recovery of pulp and filler components. They function well under both acid and alkaline papermaking conditions, that is, over a pH range of about 4-9.
U.S. Pat. No. 2,217,466 describes the early use of polysilicic acid or active silica as a coagulant aid in the treatment of raw water. The article "Activated Silica, a New Chemical Engineering Tool" by Merrill and Bolton, Chem. Eng. Progess 1 (1947), 27, summarizes the development and application of anionic active silica and mentions its use as a coagulant for paper mill white water and as a retention aid for fiber and filler fines when added to the head box of a paper machine. No mention is made of the co-use of anionic active silica together with cationic polymers.
U.S. Pat. No. 3,224,927 and U.S. Pat. No. 3,253,978 disclose the co-use of cationic starch together with anionic colloidal silica as a binding agent for inorganic fibers in refractory fiber bonding applications. The quantities of colloidal silica used are considerably larger than in papermaking applications, that is, 10-20 weight percent (wt. %) of the product for fiber bonding versus about 1 wt. % of the product for paper applications. Also, in fiber binding, conditions leading to flocculations are to be avoided whereas in papermaking, flocculation is a desired result of the additions.
U.S. Pat. No. 4,388,150 discloses a binder composition comprising colloidal silicic acid and cationic starch for addition to papermaking stock to improve retention of stock components or for addition to the white water to reduce pollution problems and to recover stock component values.
International Patent Publication WO86/00100 extends the application of colloidal silicas in papermaking to more acid conditions by describing the co-use of aluminated colloidal silica with cationic starches and cationic guars. Alumination provides stronger acid sites on the surface of the colloidal silica. As a consequence, anionic charge is maintained well into the acid range. The preferred compositions are those containing non-aggregated silica particles of relatively large 5-6 nm diameter, surface area of 500 m.sup.2 /g and an alumina/silica mole content of about 1/60.
International Patent Publication WO86/05826 describes the co-use of the above aluminated colloidal silica and cationic polyacrylamides in papermaking.